Analysts say the war in Syria is also expected to be discussed, as Iran is a key ally of President Bashar al-Assad.

It is the first visit to Iran by an EU policy chief since 2008.

'No guarantee'

"This interim agreement is really important but not as important as a comprehensive agreement [which is]... difficult, challenging," Baroness Ashton said at a joint news conference with Mr Zarif.

"There is no guarantee that we will succeed."

There was, she added, a need for support from all sides.

Mr Zarif said Iran had shown good faith and political will, adding: "We have done our side. It is up to the other side... to come to the negotiating table with a desire, decision and commitment to reach a mutually acceptable agreement."

Baroness Ashton also held talks with President Rouhani and Parliament Speaker Ali Larijani in separate meetings on Sunday.

The BBC's Lyse Doucet, who is in Tehran, says the visit is primarily aimed at exploring the potential for a new relationship between Iran and Europe.

But it is hoped Baroness Ashton can send a message to Iran's leaders that there is much to gain if they continue to build a new relationship with the international community, she adds.